1. Field of the Invention
The present invention generally relates to image recording apparatus such as copying machines and data recorders. More particularly, the present invention relates to such image recording apparatus in which unfixed images which arrive at the fixing station of an image forming process are fixed on an image supporting member such as a recording paper by applying thereto a high frequency wave.
2. Description of the Prior Art
In the art there have been already known heat fixing apparatus employing high frequency waves such as microwaves. Some examples of such fixing apparatus are disclosed in Japanese Patent Application Publication No. 38,171/1974, Japanese Patent Application Laid-open No. 20,039/1977 and Japanese Patent Application Publication No. 10,865/1979. These known high frequency heat fixing apparatus have solved various problems involved in the so-called external heat fixation. Improvements attained by these prior inventions are as follows:
The waiting time required to obtain the condition necessary for fixing was substantially reduced.
The danger of recording material such as paper catching on fire was removed. In the conventional external heat fixing apparatus, the possibility of such danger was not small in the case where the recording paper got jammed or stayed for a long time in the area of the fixing station due to some type of trouble.
Troubles associated with creasing of the recording material and/or distortion of images were removed.
To aid in a better understanding of the present invention, a further detailed description of the prior art apparatus will be made with reference to FIGS. 1 to 4 of the accompanying drawings.
FIGS. 1 and 2 show an example of the high frequency heat fixing apparatus according to the prior art wherein FIG. 1 is a perspective view thereof and FIG. 2 is a sectional view of the essential part thereof.
The apparatus uses a hollow microwave waveguide as the waveguide member 8 of high frequency application means.
Designated generally by 1 is a microwave generator for generating a microwave which has the electric field component in the transmission direction 2. Within the microwave generator 1 there is used a magnetron. 3 is a hollow waveguide of rectangular shape for transmitting the microwave in the direction of arrow 2 through the tube. The waveguide 3 is disposed in parallel to a recording material 7 carrying thereon a visualized image (toner image) indicated by 6 in FIG. 2. The recording material 7 is moved in sliding fashion on the waveguide 3 in the direction of arrow 11. During the slide movement of the recording member 7 the backside surface of the recording material faces the surface of the waveguide 3 on which a plural number of slits 4 are provided. Through the slits 4 the microwave is emitted out from the interior of the waveguide and is applied to the recording material 7 and also to the visualized image thereon. The applied microwave effects fixing of the visualized image on the recording material (more concretely, fixing is effected mainly by the phenomenon of self-heating and melting of the visualized image which absorbed the high frequency wave).
According to the prior art, the plural number of slits 4 are regularly arranged in parallel to each other and at an angle, for example, at 45.degree. relative to the recording material moving direction 11. As the slits 4 are arranged in inclined fashion relative to the recording material moving direction 11, any portion of the recording material will pass over any portion of any slit 4. Therefore, it is assured that the microwave energy will be applied to the entire surface of the recording material without any loss of the energy by leakage of the microwave.
Designated by 5 is a microwave absorber to attenuate the microwave energy generated in the waveguide 3. Since the hollow waveguide 3 is sandwiched in between the microwave absorber 5 and the microwave generator 1, the microwave energy within the waveguide is damped down and extinguished.
The fixing apparatus according to the prior art in which a hollow microwave guide is used as the high frequency application means has, however, some disadvantages for practical use in an image forming apparatus.
Firstly, it takes up a large space. Secondly, it is difficult to obtain microwave energy of high density. Thirdly, it suffers greatly from contamination dust and particles which originate from the developer and the recording material make dirty not only the outer surface of the microwave waveguide but also the interior thereof through the slits 4. Moreover, it absorbs moisture in the air and in the recording material. This dust and moisture cause a substantial loss of energy. Therefore, the waveguide requires frequent cleaning. Lastly, the edge of the recording material is often caught in the slit 4 thereby hindering the recording material from being moved smoothly.
To solve the above problems there has been proposed an improved high frequency heat fixing apparatus as shown in FIGS. 3 and 4. The apparatus has a compact construction and is operable with a higher efficiency of high frequency heating. FIG. 3 is a perspective view of the improved apparatus and FIG. 4 is a sectional view of the essential part thereof.
In this apparatus, an upper conductor plate 9, a solid dielectric substance 12 and a lower conductor plate 10 constitute a high frequency application means. As seen in FIGS. 3 and 4, the upper conductor plate 9 is flat and has a plural number of slits 4 the function of which is the same as that of the slits shown in FIGS. 1 and 2. The conductor plate 9 is, for example, a metal plate. The solid dielectric substance 12 is, for example, an alumina porcelain, steatite porcelain, white mica or ceramics. The lower conductor plate 10 may be also a metal plate and it is disposed in opposition to the upper conductor plate 9 through the dielectric substance 12. The upper and lower conductor plates 9 and 10 are adhered to the top surface and the backside surface of the solid dielectric substance 12 respectively. The size of the conductor plates 9, 10 measured in the direction of the microwave being transmitted is longer than that of the recording material. But, the length l.sub.10 of the lower conductor plate 10 measured in the direction of the recording material being moved is smaller than the length l.sub.9 of the upper conductor plate 9 in the same direction. As seen in FIG. 4, the lower conductor plate 10 is disposed in opposition to the openings of the slits 4.
The above described high frequency application means composed of a conductor plate 9 provided with slits 4, a solid dielectric substance 12 and a conductor plate 10 has particular advantages over that shown in FIGS. 1 and 2. The use of dielectric substance 12 enables the transmission of the microwave generated from the oscillator with a higher microwave density in a smaller cross-section. Therefore, it is possible to reduce the size of high frequency application means.
However, as compared with the high frequency application means shown in FIGS. 1 and 2, the high frequency application means shown in FIGS. 3 and 4 has some drawbacks. The dielectric substance in the latter is heated by the microwave during transmission and it becomes fatigued by heat in a relatively short period of time. For this reason, it is not possible to increase the output of the microwave generator very high. In addition, it is necessary to provide means for effectively cooling said high frequency application means, which in turn makes the structure of the fixing apparatus large and complicated.